Many different types of materials are utilized as functional fluids and functional fluids are used in many different types of applications. Such fluids have been used as electronic coolants, atomic reactor coolants, diffusion pump fluids, synthetic lubricants, damping fluids, bases for greases, force transmission fluids (hydraulic fluids) and as filter mediums for air conditioning systems. Because of the wide variety of applications and the varied conditions under which functional fluids are utilized, the properties desired in good functional fluid necessarily vary with the particular application in which it is to be utilized with each individual application requiring a functional fluid having a specific class of properties.
At present, there are four major classes of hydraulic fluids used in industrial hydraulic systems. These are petroleum oils, water/glycol solutions, water-in-oil emulsions and completely synthetic types. It is well known in the art that the ability of the fluid to resist fluid propagation is one of degree. Fluids of the four types mentioned have varying degrees of fire-resistance and are used in applications according to the severity of the conditions, taking into account such factors as degree of danger from fire, operating temperature, bearing loads and cost.
The term "fire-resistant fluid" as used herein means a fluid of such chemical composition and physical characteristics that it will resist the propagation of flame under certain conditions hereinafter defined.
Many synthetic fluids such as the aryl phosphate esters offer a high degree of fire resistance and are usually employed when the danger from fire is great. The cost of synthetic fluids has restricted their use to the most severe conditions. The water containing fluids, while offering an acceptable degree of fire resistance at low cost, are not desirable in systems operating at high temperatures or where good lubricity of the fluid is required or where danger from fire is great.
Petroleum oils, while offering good lubricity, are the least fire resistant but are used in many applications having a marginal fire hazard due to their low cost and general availability. Previous attempts to render petroleum oil more fire resistant by incorporating therein known fire-resistant compounds such as phosphate esters have not produced fluids having a generally acceptable combination of lubricity, fire resistance and homogeneity.
Numerous proposals have been made for correcting one or another of these properties, but correction of one property is usually effected at the expense of another property. For example, the incorporation of alkyl phosphate esters in petroleum to improve fire resistance decreases hydrolytic stability. The aryl phosphate esters, while providing superior fire resistance and hydrolytic stability, can only be added in small amounts due to the limited miscibility of the esters in petroleum oils and such amounts are ineffective in producing any significant increase in fire resistance. Also, previously, aliphatic and olefinic chlorinated hydrocarbons have been combined with mineral oil to improve fire resistance; however, they have required either the use of only minor amounts of mineral oil thus not achieving economical fire-resistant compositions or the use of significant amounts of corrosion inhibitor because such chlorinated hydrocarbons tends to be corrosive to metals. The combination of approximately equal amounts of aryl phosphate esters and chlorinated hydrocarbons yields a fluid with good flame resistance and fair lubricity, but requires the addition of VI improvers to obtain a satisfactory viscosity index.
Of the foregoing, the use of functional fluids as lubricants and hydraulic fluids, particularly industrial lubricants and hydraulic fluids, has posed a difficult area of application. Increasing demands to improve the safety of industrial manufacturing as a whole has cause the extended use of fire-resistant fluids, e.g., fire-resistant lubricants and fire-resistant hydraulic fluids in a wide range of industries.
A number of fluids are known which are intended for use to transmit power in hydraulic systems including some fluids intended for use in the hydraulic systems of aircraft. However, the hydraulic power systems of aircraft for operating various mechanisms of an airplane impose stringent requirements on the hydraulic fluid used. Not only must the hydraulic fluid for aircraft meet stringent functional and use requirements, but in addition such fluid should be sufficiently non-flammable to satisfy aircraft requirements for fire resistance. The viscosity characteristics of this fluid must be such that it may be used over a wide temperature range; that is, adequate viscosity at high temperatures, low viscosity at low temperatures and a low rate of change of viscosity with temperature. Its pour point should be low. Its volatility should be low and the volatility should be balanced; that is, selective evaporation or volatilization of any important component should not take place at temperatures of use. It must possess sufficient lubricity and mechanical stability to enable it to be used in hydraulic systems of aircraft in which conditions are severe on the fluid used. It should be chemically stable to resist such chemical reactions as oxidation, thermal degradation, and the like so that it will remain stable under conditions of use and not lose the desired characteristics, due to high and sudden changes of pressure, temperature, and contact with various metals which may be, for example, aluminum, bronze, steels, and the like. It should also not deteriorate the gaskets and packing of the hydraulic system. It must not adversely affect the materials of which the system is constructed, and in the event of a leak, should not adversely affect the various parts of the airplane with which it may accidentally come in contact. It sould not be toxic or harmful to personnel who may come in contact with it. Furthermore, in addition to all such requisites for aircraft use, the fluids must be sufficiently non-flammable to meet aircraft requirements.
The importance of attaining a hydraulic fluid that is shear stable cannot be overemphasized.
All qualified fire resistant aircraft hydraulic fluids incorporate viscosity modifiers to maintain certain minimal viscosities at prescribed operating temperatures. Since these viscosity modifiers are generally high molecular weight polymers, they are prone to mechanical or sonic shear resulting in a viscosity decrease of the fluid. Since hydraulic equipment operates most efficiently at certain specified viscosities, an excessive viscosity change can lead to less efficient performance of the system.